This invention relates to gas turbine engine propulsion systems and, more particularly, to exhaust devices of the flight maneuvering variety for use therein.
The invention herein described was made in the course of or under a contract, or a subcontract thereunder, with the United States Department of the Air Force.
The high velocity imparted to exhaust gases of a gas turbine engine by the exhaust nozzle provides thrust for propulsion. This thrust is substantially opposite to the direction of the flow of exhaust gases exiting the nozzle. Consequently, if the direction of the exhaust gases is changed, the direction of propulsive thrust is correspondingly varied. Typically, aircraft gas turbine engines are provided with nozzles which are fixed in the axial direction, and aircraft maneuvering is accomplished solely by airframe control surfaces. Advanced aircraft configurations contemplate, and may even require, the selective redirection (or vectoring) of gas turbine engine thrust in order to enhance aircraft performance and to provide the aircraft with operational characteristics heretofore deemed impractical. For example, if the exhaust of a conventionally installed gas turbine engine is directed downwardly, rather than rearwardly, to a direction substantially perpendicular to the engine longitudinal axis, the resulting upward thrust would provide direct lift for the aircraft and, if properly controlled, a vertical take-off and landing capability. Similarly, thrust vectoring during flight can greatly increase aircraft maneuverability since the thrust force can augment the maneuvering forces of the aircraft control surfaces such as elevators, ailerons, and rudders. In order to obtain such in-flight maneuverability, a device is required to efficiently and practically alter the direction of gas turbine engine exhaust nozzle gases.
Thrust vectoring may be employed in essentially two types of applications. First, it may be used in vertical take-off and landing (VTOL) applications where aircraft operation is at low speed and where continuous vector angle capability up to essentially 90.degree. is required for generating aircraft lift. Secondly, thrust vectoring is employed at relatively high aircraft speed to achieve combat maneuver capability, the range of vectoring being limited to approximately 30.degree. or 40.degree.. The fundamental difference between these two concepts is that the VTOL application generates system lift by simply deflecting the engine flow, whereas an in-flight application utilizing the principle of supercirculation provides lift augmentation that is several times greater than the vertical thrust component of the VTOL application. As is well known in the art, supercirculation refers to the additional wing lift generation due to directing flow out of or over a wing in such a manner as to effectively change the aerodynamic shape of the wing. This lowers the required angle of attack at high subsonic maneuvering conditions, thus enabling the aircraft to make high "G" turns with less drag.
It is predicted that drag reductions in excess of 40% are attainable at typical combat conditions when an aircraft incorporates a flight maneuverable propulsion system for lift augmentation wherein the engine exhaust flow is expelled past the wing in such a manner as to provide the additional lift through supercirculation. This drag reduction allows the engine to be sized significantly smaller than otherwise possible.
Many types of thrust vectoring nozzles have been studied in the past. For example, the three-bearing hinge flap nozzle taught in U.S. Pat. No. 3,687,374, D. O. Nash, entitled "Swivelable Jet Nozzle," and which is assigned to the assignee of the present invention, is a conventional axisymmetrical nozzle supported on a duct equipped with three rotatable bearings to achieve thrust deflection. It is primarily a V/STOL nozzle and not readily adaptable to in-flight maneuver vectoring due to rather large drag-producing base areas while in the deflected mode. The block-and-turn type of swivel nozzles as depicted in U.S. Pat. No. 3,035,411, C. P. Porowski, and U.S. Pat. No. 3,837,411, D. O. Nash et al, both of which are assigned to the assignee of the present invention, require a flow-diverting valve for selection between the cruise nozzle and the V/STOL nozzle. Such a valve in combination with two exhaust nozzles results in a heavy structure which does not integrate well with an aircraft wing flow field when it is desired to take advantage of the supercirculation effects.
An important consideration in the development of an efficient propulsion nozzle is control of the flow path throat area (area of minimum cross section) and the area at the discharge of the nozzle. The throat area is normally defined by a convergent/divergent portion of the nozzle. A convergent section of the nozzle is designed to keep the turbine discharge gases subsonic until they reach the throat, at which time they reach a sonic velocity. A divergent portion subsequently allows controlled expansion of the gases which permits their velocity to become supersonic. In addition to the absolute value of the flow path throat area and the nozzle exit area, the ratio of the latter to the former is a significant parameter governing thrust propulsive efficiency. The exhaust system design greatly influences the overall engine performance and the choice of nozzle areas is determined by turbine inlet temperature, mass airflow, and the velocity and pressure of the exhaust stream. While little is to be gained by use of variable area nozzles in low performance aircraft, in high performance aircraft significant operational advantages can be realized.
Typically, the variable area nozzle is opened during low altitude take-off and closed thereafter, at an appropriate altitude, in order to obtain necessary cruise thrust. The nozzle is usually automatically controlled on a predetermined schedule by the main engine control as required by the flight environment. The addition of an in-flight thrust vectoring capability to gas turbine engines introduces a new dimension into nozzle area controls and means must be devised to essentially override the "nominal" area scheduling in order to produce optimum nozzle areas during the thrust vectored (or flight maneuvering) mode. Such a system should be as simple as possible consistent with the requirements of a fail-safe design.
Furthermore, nozzles which cooperate with aircraft wing and flap structure are inherently wide (i.e., possess a large span). Accordingly, a large area is available upon which the exhaust gases act, thereby requiring large flap actuation forces and heavy actuators. Since weight is always of paramount consideration in aircraft component design, a means is desired to reduce actuation loads and actuator weight.
The problem facing the gas turbine engine and aircraft designers, therefore, is to provide a flight maneuverable propulsion nozzle which minimizes flow turning losses, is matched to the aircraft wing to take advantage of the effect of supercirculation, provides adequate nozzle area control and which can be manipulated with a minimum of actuators.